Typical known coatings for automobile mirrors consist of the highly reflecting metals silver and aluminium. The reflection values achievable amount to more than 85% in the visible light spectrum. Although, however, high reflection values are desirable in the daylight, they lead at night to a glaring of the driver through the headlights of the following vehicles. Therefore, mirror coatings were developed in the past, which are destined to reduce the glaring effect when driving at night.
One possibility of reducing the glare is the use of alternative reflector materials, such as chromium or titanium chromium (double-layer system of chromium on titanium, see DE 197 390 46). With this, mirrors with a neutral reflection and a reduced reflectance of approx. 45% to 60% can be realized. A further reduction is impossible at present, due to statutory provisions, for example, ECE regulation No. 46, page 20, 6.1.2.2.5 (40%) and “FMVS111, page 329, S11 (35%),which prescribe a minimum reflection of 40% in the daylight.
Afterwards, it was tried to improve the behavior of the mirrors through spectrally selective reflection coatings. EP 0176935 B1, for example, describes a blue rearview mirror. Blue mirrors reflect wavelengths in the red or green spectrum to a lesser extent than wavelength in the blue spectrum. Because the spectral composition of the light of the headlights usual at the moment of application of the patent EP 0 176 935 B1 had a high red contribution and a low blue contribution, the blue mirrors additionally diminished the (glaring) light of these headlights.
To evaluate the spectral properties of the mirror coatings, one must in general take into account that the response of the human eye differs greatly within the visible spectral region. This is represented by the so-called response characteristic V(λ), λ designating the wavelength of the light. V(λ) is valid, however, only for the bright adaptation of the eye. With decreasing brightness of the environment, this characteristic shifts towards shorter wavelengths up to the dark-adapted response characteristic V′(λ).
Therefore, another approach for the development of low-glare mirrors were mirrors which, at a relatively high level of spectral reflectance, reduced in particular the region of the highest eye responsiveness at wavelengths of approx. 550 nm (U.S. Pat. Nos. 4,921,331, 4,805,989 and 4,955,705).
With an alternative technology to be mentioned here, one tried to reduce the risk of glaring through the headlights of following vehicles by means of the fact that the mirror structure contains parts of variable transmission, switched, e.g., electrically (e.g. electrochromic mirrors or LCD mirrors). The before-mentioned technologies involve, however, a high expenditure. They require in particular measures of circuit technology to ensure that the low reflection values achievable with them are only effective in concrete cases of acute glare. Accordingly, only vehicles from the top segment are equipped with them at present.
Another important variable for evaluation is the so-called color rendering index Ra, following the standard DIN EN 410, which is a measure for the manner in which colors can be reproduced or distinguished in the reflected mirror image. The color reproduction index is relevant in particular for visual recognition of the road traffic happenings. Briefly, systems with neutral reflection have a good color rendering (Ra almost 100), while the color reproduction index of intensely colored mirrors is clearly reduced. This limits in practice the influencing of the glare behavior by means of the mirror spectrum.
The optimum reduction of glare is made even more difficult through the fact that meanwhile other light sources to be used in motor-vehicle headlights were developed, which spectrally differ clearly from the known light sources (halogen headlights) and which complicate the relations.